1. Field of the Invention
This invention relates to a camera in which a photo-taking optical system is moved in the direction of the optic axis by a drive source such as a motor and the optical system can be set at a plurality of positions.
2. Related Background Art
A camera of this type has, for example, a guide cylinder rotatably supported on the camera body and formed with helicoid in the inner surface thereof, a lens barrel having a helicoid meshing with the helicoid of the guide cylinder formed in the outer surface thereof and having a photo-taking lens disposed therein, a drive motor for rotatively driving the guide cylinder, and an encoder operatively associated with the guide cylinder, and is such that the position of the lens barrel in the direction of the optic axis is known from a position signal from the encoder and the lens barrel is moved to a predetermined photographing position.
Now, the applicant has proposed in U.S. Pat. Nos. 4,597,657, 4,643,555 and 4,669,848 cameras provided with three positions, i.e., the reset position in which the lens barrel has been substantially fully brought into the camera body, the wide angle photographing position in which the lens barrel has been axially moved to the intermediate position in the direction of the optic axis, and the telephoto photographing position in which the lens barrel has been most axially moved. In these cameras, the position of the lens barrel is controlled in the following manner.
A substrate formed with an electrically conductive pattern is secured to the guide cylinder, and for example, three terminals slidable on the electrically conductive pattern are provided on a camera side member fixed to the guide cylinder to constitute an encoder, and a position signal corresponding to the position of the lens barrel is obtained from each terminal of the encoder. The guide cylinder is rotatively driven by the drive motor to axially outwardly or inwardly move the lens barrel through the two helicoids. The position command signal of the lens barrel output from a change-over switch such as a wide angle and telephoto selecting switch and the position signal from the encoder are compared with each other, whereby the position of the lens barrel is known, and the drive motor is stopped at a predetermined position and the lens barrel is directed to the predetermined position.
However, there is always back-lash between the helicoid on the drive side and the helicoid on the driven side. This has led to the problem that a positional error occurs during the axially outward movement of the lens barrel and during the axially inward movement of the lens barrel.
Also, each gear of a gear train forming the transmission route leading from the motor to the guide cylinder is resiliently deformed during the transmission of the drive force for the movement of the lens barrel, and accumulates therein a force of restitution corresponding to the amount of resilient deformation still after the completion of the movement. This force of restitution always acts on the already aligned lens barrel and therefore, there has been the undesirable possibility of the lens position being inadvertently shifted by an extraneous factor such as vibration or the like.
On the other hand, when the lens barrel is to be moved by the motor and stopped at a predetermined target position, for example, the zooming position for obtaining a desired focal length or the in-focus position obtained as a result of focus detection, such speed control that will decrease the movement speed as the lens barrel approaches the target position is preferable from the viewpoints of the saving of the time required for the movement and the accuracy of the stop position. Such control is known, for example, U.S. Pat. No. 4,238,143, etc. This prior art is such that when the photo-taking lens is to be moved in the direction of the optic axis and stopped at a predetermined position, a DC voltage is first supplied to the motor and after the lens has come near the target position, a pulse voltage is supplied to the motor to control the photo-taking lens at a low speed.
However, even in such control, in a camera using a battery as a power source, the power source voltage is reduced with the time of use of the battery and therefore, the rotational speed of the motor also fluctuates. Particularly, if the speed during the low speed control in the vicinity of the target position is not constant, the influence of the inertia force will differ, and the amount of movement of the lens taking place from after the power supply to the motor is stopped until the motor is completely stopped will also differ. This has also led to the problem that the accuracy of the stop position is not stable.